Expanded thermoplastic resin beads are excellent in lightness in weight, cushioning properties, and heat insulating properties, and expanded beads molded articles having a three-dimensional shape are obtained by means of in-mold molding. The expanded beads molded articles are high in a degree of freedom of shape design, so that they are utilized as a cushioning material, a container, a heat insulating material, a vibration-damping material, or the like in multipurpose fields inclusive of packaging fields, commodities for living, building and civil engineering materials, vehicle members, and the like. The expanded thermoplastic resin beads are roughly classified into those made of a styrene-based resin as a base material resin and those made of an olefin-based resin as a base material resin. Among them, the expanded beads made of a styrene-based resin as a base material resin are used more frequently than the expanded beads made of an olefin-based resin as a base material resin for the reasons that the former is excellent in a balance between lightness in weight and compression strength of the expanded beads molded article, is easy for undergoing in-mold molding of the expanded beads, and is inexpensive in terms of a raw material. But, though the expanded beads made of a styrene-based resin have excellent aspects as described above, they are insufficient in heat resistance, chemical resistance, durability, toughness, and creep resistance characteristics depending upon uses. For that reason, among the expanded beads made of an olefin-based resin, the expanded beads made of a propylene-based resin, which are excellent in heat resistance, chemical resistance, durability, toughness, creep resistance characteristics, and so on, are watched.
In addition, since the expanded propylene resin beads molded articles having voids are excellent in characteristics, such as water permeability, air permeability, sound-deadening qualities, etc., and have appropriate rigidity and elasticity, they are used for building and civil engineering materials, vehicle interior materials, and so on, and their uses are expanded. Furthermore, following the recent expansion of uses, performances required against the expanded propylene resin beads molded articles having voids are being more increased. That is, from the viewpoints of enhancements in characteristics, such as water permeability, air permeability, sound-deadening qualities, etc., expanded beads molded articles having a high voidage are required. Moreover, from the viewpoints of durability, impact resistance, and the like, expanded beads molded articles which are strong in fusion strength among the expanded beads and also excellent in deflection properties are required.
But, there is involved difficulty in the aspect of molding processing, for example, a molding pressure of a heating medium, such as steam, etc., is high due to crystallinity and heat resistance of the propylene-based resin, and hence, an improvement of the in-mold molding method using expanded propylene resin beads is required. In addition, from the standpoint of performances of an expanded beads molded article, the requirements for lightness in weight and high impact energy absorbing performance are increasing.
As the conventional art, for example, PTL 1 discloses that in order to utilize fusion characteristics by a propylene-based resin having a low melting point, thereby decreasing a molding pressure of steam at the time of in-mold molding, while revealing appearance, heat resistance, and mechanical physical properties by a propylene-based resin having a high melting point, the propylene-based resin having a high melting point and the propylene-based resin having a low melting point are mixed under specified conditions. In addition, for example, PTLs 2 and 3 disclose that in order to decrease a molding pressure of steam at the time of in-mold molding of expanded propylene resin beads, expanded propylene resin beads in which a core layer of the expanded bead is covered with a resin having a low melting point are used.